True enough that at the time God spoke earth into existence the was no concept of time. However as he created the earth, he established the week and here showed us how to mark time. 6 solar days and one day of rest in a week. The Jewish culture marks the Sabbath on Saturday, this then puts the first day of the week on Sunday. No one knows the exact day that God created the earth, but our week models that week. 

No in the beginning refers to a time when there was no recorded time such as 12 oclock or monday. These days that we refer to come to us from the roman empire which worshiped other gods like the sun and moon or saturn ect. thats where the words sunday, monday ,tuesday saturday come from. Today we know that the sun and moon are not gods and its just another name for a day.